William Shakespeare (1594) in "Sow'd cockle, reap'd no corn" (Love's Labour's Lost, Act IV, Scene 3) refers to the seed gall nematode. The number of common names used for nematodes also indicates their importance to people.

The phylum Nematoda includes an enormous variety of free-living and parasitic species. Nematodes can be free-living and parasitic in plants, vertebrates, and invertebrates. It is thought that the ancestors of nematodes were free-living, but at different times, several different nematode branches have become parasites. Because there have been multiple invasions of animals by descendants of free-living forms, parasitic and free-living nematodes occur throughout the phylum, often within related taxonomic groups.

The largest known nematode, Placentonema gigantissima, lives in the placenta of the sperm whale and is up to 30 feet (8-9 meters) long, with a body volume of 174 Litres! The nematodes - parasites of mammals and insects can be many centimeters long. On the contrary, most of the nematodes living in the soil are very small - normally 0.3 to 3 mm long as adults, the immature ones are even smaller. In fact, if you can see it with the naked eye, it is probably a mermithid or a Gordian worm, and not a "true" soil nematode. The Mermithida (phylum Nematoda) and the Gordian worms (phylum Nematomorpha) are insect parasites - the juveniles grow inside the body cavity of an insect (such as weta), and the adults come out when they are mature, killing the insect in the process. Mermithids are not soil animals in a true sense, but can be found in the soil occasionally. Gordian worms are the long (20+ cm), slow-moving "horse hairs" that people find in running water. Mermithids and Gordian worms are harmless to humans.

Nematode under a compound microscope, New Zealand.

All nematodes are essentially aquatic animals, which live and move in fluids. Soil nematodes live in water films surrounding soil particles, although many are able to form resistant dispersal forms and are phoretic on mites and insects. The body of a nematode is long and thin (worm-like), circular in cross-section, and normally not coloured. The body wall is translucent, and the colourless body contents (the tubular gut and the gonads) can be seen through under a microscope. Males and females generally look similar. The identification of nematodes is mostly based on the shape of their feeding apparatus and the oesophagus, reproductive structures, and the tail shape. The most obvious character, which separates nematodes from other small worms, is the absence of segmentation - none of the organs are repeated on the sequential basis.

Another character is the locomotion of live nematodes - they are unable to expand or contract their body longitudinally, and move in an undulatory, sinusoidal fashion. Enchytraids (pot worms) can be mistaken for nematodes - but pot worms are larger (0.5-1.0 cm long), white, and segmented, which can be seen under magnification.

Notes on biology

Unless we look at it closely under a microscope, it is impossible to tell if a nematode from a soil sample is parasitic or free-living. In the soil, there are many free-living nematodes that feed on soil microbes or are predators - these nematodes complete their entire life cycle in the soil. In addition, many nematodes that are parasites of invertebrates, vertebrates, and plants have free-living juvenile stages in the soil. For example, the nematode parasites of sheep and cattle have a free-living soil stage that feeds on bacteria! When these nematodes are ready to infect the vertebrate host, they climb a blade of grass and get eaten by a sheep or a cow. The hookworms - parasites of dogs, cats, and humans, also have a free-living soil stage, although, fortunately, human hookworm is not endemic to New Zealand. Similarly, the juvenile stages of insect parasites live in the soil as well. So in a soil sample from New Zealand pasture, one will not find the parasites of humans, but one might find the free-living juveniles of the parasites of sheep, cattle, insects and plants, as well as the free-living adults of bacteria-feeding nematodes, fungi-feeding nematodes, and predatory nematodes.

The shape of the feeding apparatus is the key to what the nematode feeds on, and there is a wide range of mouth structures. A very narrow mouth opening with no projections or with small feather-like projections indicates a nematode that feeds on bacteria. The "hypodermic syringe" feeding structures are found in the nematodes that feed on fungi, plant cells, are predators, or insect parasites. Many predatory nematodes feed on other nematodes, and there is a whole genus that feeds on enchytraeids.

Another soil nematode.

In turn, many soil animals feed on nematodes themselves. Tardigrades and mites are predaceous on nematodes. There are also fungi that feed on nematodes. Some of them parasitise and kill living nematodes, or are saprophageous on dead nematodes. There is also a group of trapper fungi - they form small loops and rings on the mycelium (body of the fungus) and trap active nematodes. The earthworms and other invertebrates may consume nematodes accidentally, while eating organic matter. These accidentally swallowed nematodes will be digested, unless they are juveniles of the parasitic species, waiting to be swallowed - although it is unknown whether these parasitic nematodes wait to be eaten or actively burrow into their hosts' bodies through the cuticle.

In most nematodes the sexes are separate, but while in some groups males are always present, in many plant and soil nematodes males are not common. If males are present, the nematodes mate - the male wraps its tail around the female transversely and inserts paired copulatory spicules into the female genital opening. In the absence of males nematodes reproduce parthenogenetically. Following the egg, there are normally four juvenile stages separated by moults - nematodes have cuticle and moult very much like arthropods. There is no metamorphosis.

Ecologically, nematodes are extremely important. A few of the soil nematodes are plant pathogens, and their impact is particularly significant when the plant is subjected to nutrient and moisture stresses, as well as nematode stress. Most animal groups have nematode parasites. Mites and Collembola are an exception - they are a little too small to have nematode parasites (the smallest nematode adults are about 0.3 mm long), but there are a few. Many nematodes infect insect pests - for example, grass grub larvae in the soil get infected with nematodes. Some of the insect-parasites are the vectors for the bacteria that kill the insects. There are nematodes that live in the body cavity of earthworms, and there are also bacteria-feeding nematodes living in nephridia ("kidney") of the earthworms. There are even records of nematode parasites of nematodes!

Most of the soil nematodes are microbial feeders, feeding on bacteria in particular. By grazing on bacteria, they keep the bacterial population turning over, so bacteria have a greater nutrient content and are more efficient at breaking down whatever they are breaking down. Also, nematodes ingest more nitrogen than they need, so the excess nitrogen is excreted. It was demonstrated in a simple system, that nematodes are increasing the rate of nutrient turnover, the nutrient availability to plants, and can stimulate plant growth significantly under good circumstances. On the other hand, increased nutrient availability to plants may not result in increased plant growth, but instead in increased populations of plant-feeding nematodes. In one experiment, the superphosphate application to a pasture was increased by a factor of 4, but the grass production did not increase at all. The scientific investigation showed that the population of root lesion nematode increased by a factor of 4, which reduced the life of the roots and increased root turnover. As a result, the plants were putting all their energy into producing new roots, instead of directing energy into shoots. By applying the fertilizer, farmers were actually feeding the nematodes below ground.

Interestingly, a nematode - Caenorhabditis elegans, or C. elegans - was the first multicellular organism for which the entire genome was sequenced.

Where to find them

Nematodes are everywhere - in soils, in marine and freshwater sediments, in the tissues of plants and in the bodies of animals. They cannot swim by themselves, and they cannot fly by themselves, but by parasitizing fish, whales, birds and insects, they can, in fact, swim and fly. Nematodes are the most numerous multicellular animals on Earth. In New Zealand pasture soil, for example, every square meter of soil has on average several million nematodes in the top ten cantimeters. In the deserts there are fewer nematodes in the soil, but they are still present. The only place without nematodes is the ice of Antarctica and mountain glaciers, where there is nothing to feed on. But as long as there are plants or bacteria for nematodes to feed on, they will be there. Even the rocky outcrops with a few lichens in Antarctica, which are thousands of kilometres away from other rocky outcrops, have nematodes on them.

Soil nematodes are aquatic animals which move in water films - the best way to extract them from the substrate is to suspend the sample (soil, litter, compost, etc.) on tissue or cheese cloth in a water-filled filter funnel or similar container. Moving randomly, nematodes just by chance will crawl through the tissue and sink to the bottom of the water container. Hundreds of nematodes can come out from just a gram of soil. This method works well for free-moving nematodes, but parasitic stages living in plant roots will not come out this way. Tap water, as long as there is not too much chlorine, can be used. Make sure that the water did not pass through new copper pipes - copper is highly toxic for nematodes.

Another good way to find nematodes for observation is to collect a dead earthworm from an urban sidewalk, submerge it in a thin layer of water in a Petri dish, and open the body cavity - there may be a swarm of nematodes in there.

Distribution and conservation

We know very little about the native fauna of nematodes in New Zealand. On the last count in 2000, there were 631 named species of nematodes in New ZealandZ - that is including soil, marine, and parasitic nematodes. The total number of yet unnamed nematodes could very well be 10,000-60,000 species - for example, every native earthworm species probably has its own nematode parasite, and there are 180 species of earthworms. There are hundreds of nematode species waiting to be described. Longidorus waikouaitii - a plant feeding soil nematode recently described from a forest remnant in the South Island of New Zealand north of Dunedin, is on the list of nationally critical species; 50+ other New Zealand nematodes are on the list of threatened and vulnerable species. The New Zealand Department of Conservation lists these species as "data deficient" - we know very little about these species.

Improved production techniques have rendered some nematode species rare. For example, Turbatrix aceti, the vinegar eelworm, is a nematode that used to live in raw vinegar made the traditional way - through fermentation. Today, the vinegar is produced mechanically and pasteurized, so the vinegar eelworm has become a rare species. Similarly, with modern seed cleaning techniques the seed gall nematode has nearly vanished - two or three species of these nematodes used to be a major problem, and are now exterminated in New Zealand.

Florida Nematology, Dept. of Entomology and Nematology, University of Florida. History of nematology, description of some nematodes-parasites of insects, links to other websites.

Plant and Insect Parasitic Nematode Home Page, University of Nebraska-Lincoln Nematology Lab. A comprehensive site, which includes an overview of the phylum, an interactive diagnostic key to plant parasitic, free-living and predaceous nematodes, an annotated list of world genera (including some species), and photo gallery.

Insect Parasitic Nematodes, by Dr. P. Grewal,
Dept. of Entomology, The Ohio State University. Includes information on biology, habitat, and life cycles of insect parasitic nematodes, many of which are found in soil.